A. Field of the Invention
The present invention relates to polymeric membranes that have been treated with ultra-violet (UV) radiation and thermal treatment. The membranes have improved permeability and selectivity parameters for gas, vapor, and liquid separation applications. In particular embodiments, the treated membranes are particularly useful for C2 or C3 olefin/paraffin separation applications.
B. Description of Related Art
A membrane is a structure that has the ability to separate one or more materials from a liquid, vapor or gas. It acts like a selective barrier by allowing some material to pass through (i.e., the permeate or permeate stream) while preventing others from passing through (i.e., the retentate or retentate stream). This separation property has wide applicability in both the laboratory and industrial settings in instances where it is desired to separate materials from one another (e.g., removal of nitrogen or oxygen from air, separation of hydrogen from gases like nitrogen and methane, recovery of hydrogen from product streams of ammonia plants, recovery of hydrogen in oil refinery processes, separation of methane from the other components of biogas, enrichment of air by oxygen for medical or metallurgical purposes, enrichment of ullage or headspace by nitrogen in inerting systems designed to prevent fuel tank explosions, removal of water vapor from natural gas and other gases, removal of carbon dioxide from natural gas, removal of H2S from natural gas, removal of volatile organic liquids (VOL) from air of exhaust streams, desiccation or dehumidification of air, etc.).
Examples of membranes include polymeric membranes such as those made from polymers, liquid membranes (e.g., emulsion liquid membranes, immobilized (supported) liquid membranes, molten salts, etc.), and ceramic membranes made from inorganic materials such as alumina, titanium dioxide, zirconia oxides, glassy materials, etc.
For gas separation applications, the membrane of choice is typically a polymeric membrane. One of the issues facing polymeric membranes, however, is their well-known trade-off between permeability and selectivity as illustrated by Robeson's upper bound curves (see L. M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci., 62 (1991) 165). In particular, there is an upper bound for selectivity of, for example, one gas over another, such that the selectivity decreases linearly with an increase in membrane permeability. Both high permeability and high selectivity are desirable attributes, however. The higher permeability equates to a decrease in the size of the membrane area required to treat a given volume of gas. This leads to a decrease in the cost of the membrane units. As for higher selectivity, it can result in a process that produces a more pure gas product.
A majority of the polymeric membranes that are currently used in the industry fail to perform above a given Robeson's upper bound trade-off curve. That is, a majority of such membranes fail to surpass the permeability-selectivity trade-off limitations, thereby making them less efficient and more costly to use. As a result, additional processing steps may be required to obtain the level of gas separation or purity level desired for a given gas.